Chladni figures

A rigid plate will have a set of natural resonance frequencies just like a spring, and when the plate is excited at one of these frequencies, it will form a standing wave with fixed nodes. These nodes will form lines on the plate, in contrast to points on the string. Chladni realized that sand sprinkled on the top of the plate would be pushed away from the vibrating regions and settle into these nodes, allowing the node patterns to be seen.

Chladni excited these resonant vibrations by drawing a violin bow across its edge, as shown above. Today, however, we can mechanically drive the plate from below with a speaker hooked up to a frequency generator (or downlaod frequency generator app for a PC).